Among others the following definitions and abbreviations will be used in the description below.
ASIC Application Specific Integrated Circuit
CPICH Ec/No The received energy per chip divided by the power density in the band
CPICH RSCP Common Pilot Channel Received Signal Code
Power
DRX Discontinuous Reception
DTX Discontinuous Transmission
eNB E-UTRAN NodeB
E-UTRA Evolved UTRA
E-UTRAN Evolved UTRAN
FDD Frequency Division Duplex
LTE Long Term Evolution
MAC Medium Access Control
RRC Radio Resource Control
RRM Radio Resource Management
RSRP Reference Signal Received Power
RSRQ Reference Signal Received Quality
SINR Signal to Interference plus Noise Ratio
UE User Equipment
UTRA Universal Terrestrial Radio Access
UTRAN Universal Terrestrial Radio Access Network
TTI Transmission Time Interval
WCDMA Wide band Code Division Multiple Access
Universal Terrestrial Radio Access Network (UTRAN) is a conceptual term that identifies that part of the network which consists of Radio Network Controllers (RNCs) and Node Bs. This Communications network is commonly referred to as 3G. Evolved UTRAN (E-UTRAN) is an evolution of the 3G radio access network towards a high-data rate, low-latency and packet-optimised radio access network.
The document 3GPP TS 36.300, V 0.3.1 (2006-11) provides an overview and overall description of the E-UTRAN radio interface protocol architecture.
The E-UTRAN is fundamentally a packet oriented system. This means that users transmit and receive data in non-continuous fashion. One important implication is that at a given time not all the users are active (i.e. not all the active users communicate simultaneously). This inherent characteristic of packet transmission is intended to be fully exploited in E-UTRAN system.
One important aspect of packet transmission is discontinuous transmission (DTX) and discontinuous reception (DRX). The E-UTRAN is primarily a packet-oriented system without any circuit switched transmission. This means that E-UTRAN can easily be optimized for packet transmission. One important feature introduced in E-UTRAN is the possibility of UE entering into DRX mode. In E-UTRAN there are three UE states with their context stored in the core network:                LTE_DETACHED        LTE_IDLE        LTE ACTIVE        
However, only the last two states also have RRC context. Therefore, from radio resource allocation and usage perspective the last two states are the most interesting ones and are therefore further described below:
In the LTE_IDLE state the UE listens to the network (e.g. by means of paging information) only at DRX instant and performs downlink measurements according to the assigned DRX cycle length and autonomous cell reselection. In order to receive data the UE needs to enter into LTE_ACTIVE state, which is an essential feature in all cellular systems to allow UE battery saving.
In the LTE_ACTIVE state the UE is able to receive data and transmit data at any time. This implies that the network maintains the full RRC context on cell level in order to be able to schedule the user whenever required. (For this reason this state is also called RRC connected state on RRC level). The LTE_ACTIVE state has a sub-state called the DTX/DRX mode for the purpose of saving UE battery consumption in case of low activity. After a certain period of inactivity the UE enters into this sub-state and starts monitoring the downlink transmitted information only at a regular interval according to the network assigned DRX cycle. The DRX cycle in this case can vary typically between 5 ms up to 1-5 seconds. However, in case of much longer inactivity of packets the UE should preferably enter into idle mode.
The advantages of the type of DRX/DTX mode described above are that, on the one hand, it allows efficient UE battery savings while, on the other hand, the network can quickly schedule a UE without unnecessary delays caused by the formalities of the call setup procedure. The concept of the DRX/DTX sub-state in RRC connected state is also used in UTRA in continuous packet connectivity, which comprises purely packet oriented transmission.
Downlink scheduling information is used to inform the UE how to process the downlink data transmission. The scheduling information may include the UE identity, resource assignment, duration of assignment, modulation scheme, etc. and may be sent on a shared control channel, which comprises of mainly layer 1 (physical layer) and layer 2 (MAC layer) contents. Generally, this information is sent in every sub-frame (0.5 ms) or at least every TTI (e.g. 1 ms). Therefore, a UE in LTE_ACTIVE mode may have to monitor this information in every sub-frame or TTI depending upon the periodicity of the transmission of scheduling information.
The handover in LTE-ACTIVE state is network-controlled. This means that the UE reports measurements to the network in response to an event. An event occurs when one or more parameters take on a certain value or certain values. En event can for instance be configured to occur when one parameter reaches a certain value individually, or when one parameter reaches one value and another parameter reaches another value, i.e. as a combination of different parameter values. The event and/or measurement reporting either triggers idle gaps for more measurement (e.g. on other carrier frequency or other technologies) or leads to handover. During an idle gap the UE tunes its receiver to another E-UTRA carrier frequency or to a carrier frequency of another access technology (e.g. UTRA or GERAN) for performing the measurements. While performing such measurements the UE does not receive or transmit any data or signaling information on the serving E-UTRA carrier frequency. In the case of handover the UE receives a handover command from the network. In DRX mode in LTE-ACTIVE state the UE can receive any network information (e.g. scheduling of handover command) only at the DRX instant, i.e. when the UE becomes active. In case of long DRX cycle (e.g. 2 or 3 seconds) the handover command reception can be delayed. But to ensure good system performance (low handover latency) both the UE measurement reporting and handover command reception should not be delayed. At the same time the characteristics of DRX mode should be retained to ensure efficient UE battery consumption.
In E-UTRAN the UE reports in the LTE-ACTIVE state the configured events and the corresponding downlink measurements when the network configured event criteria or conditions are fulfilled. As indicated above, the UE can also, in an LTE_ACTIVE state, operate in DRX mode in which the UE monitors the downlink scheduling channel only at DRX instances. In order to prevent handover delay, it has been agreed that at the occurrence of an event the UE shall report both the measurement quantity and the event as quickly as possible without waiting for the next active period.
Another important aspect is the delivery of the handover command in response to the event triggered measurement reporting. Since the UE listens periodically only at DRX instances, this can unnecessarily delay the handover command reception especially in case of long DRX cycle. But in order to ensure low handover latency the handover command, whenever required, should be delivered to the UE as soon as possible. It has therefore also been agreed that the UE shall interrupt its DRX activity after each and every event triggered measurement report and start monitoring the scheduling control channel either continuously or according to a pre-configured shorter DRX cycle. This will allow the network to schedule the UE immediately for the purpose of sending a handover command. One major drawback with this approach is that in case the network does not send handover command the UE battery will be inefficiently utilized.
There are several reasons why the network may not send a handover command in response to an event-triggered measurement report:                The network may set lower threshold to get more frequent events for monitoring network performance (e.g. operational and maintenance issues).        Another reason is that event triggered report may not directly lead to a handover; rather this may trigger gap-assisted measurements {e.g. measurements on another frequency or on another access technology).        A particular UE measurement report may also be utilized for RRM functions other than handovers such as congestion control etc.        Depending upon the implementation not all measurements may trigger handover or at least some measurements may not directly trigger the handover. As an example in WCDMA the UE transmitted power reporting (and corresponding event) is used in the network to trigger the compressed mode. UTRAN can command that the UE enters into compressed mode depending on UE capabilities, which define whether the UE requires compressed mode in order to monitor cells on other FDD frequencies and on other modes and radio access technologies.        Depending upon a particular implementation the network may perform handover according to combination of measurements, e.g. based on UE transmitted power and downlink received SINR.        
Thus, there are a number of cases in which the network does not send a handover command in response to an event triggered measurement report, whereby the following control channel monitoring leads to excess battery consumption in the prior art solution.